iodine

iodine (I), chemical element, a member of the halogen elements, or Group 17 (Group VIIa) of the periodic table.

History

In 1811 the French chemist Bernard Courtois obtained a violet vapor by heating seaweed ashes with sulfuric acid as a by-product of the manufacture of saltpeter. This vapor condensed to a black crystalline substance, which he called “substance X.” In 1813 British chemist Sir Humphry Davy, who was passing through Paris on his way to Italy, recognized substance X as an element analogous to chlorine; he suggested the name iodine from the Greek word ioeides, “violet colored.”

Occurrence and distribution

Iodine is never found in nature uncombined, and it is not concentrated sufficiently to form independent minerals. It is present in seawater, but sparingly, as the iodide ion, I⁻, to the extent of approximately 50 mg per metric ton (0.0016 ounce per ton) of seawater. It is also formed in seaweeds, oysters, and cod livers. Sodium iodate (NaIO₃) is contained in crude Chile saltpeter (sodium nitrate, NaNO₃). The human body contains iodine in the compound thyroxine, which is produced in the thyroid gland.

The only naturally occurring isotope of iodine is stable iodine-127. An exceptionally useful radioactive isotope is iodine-131, which has a half-life of eight days. It is employed in medicine to monitor thyroid gland functioning, to treat goitre and thyroid cancer, and to locate tumors of the brain and of the liver. It is also used in investigations to trace the course of compounds in metabolism. Several iodine compounds are used as contrast mediums in diagnostic radiology. In aqueous solution even minute amounts of iodine in the presence of starch produce a blue-black color.